1. Field of the Invention:
This invention relates to a camera in-focus detecting device which is arranged to permit checking the action thereof.
2. Description of the Prior Art:
Many focusing methods have been proposed. In a so-called a TTL focusing method the focus a photo-taking optical system is detected by guiding light passing through the photo-taking optical system to a photo-electric focus detecting device. In placing such an in-focus detecting device within a single-lens reflex camera, the detecting device must be arranged in a limited space available there. Therefore, it has been a general practice to dispose it beneath a movable mirror of the camera. Meanwhile, since the movable mirror is subject to repetitive upward and downward moving operations, air about the mirror tends to be fraught with minute dust and the like. Such dust in the air then tends to stick to the light receiving surface of the detecting device. In such a case, the sticking dust or the like affects the in-focus detecting device and results in generation of an erroneous signal.